In general, electrically conductive coating, film, or sheet is produced from a mixture containing electrically conductive material and paint or film material. Widely used electrically conductive materials include metallic powder, electrically conductive inorganic oxide powder, and carbon powder. However, metallic powder has a drawback in that the electrical conductivity of the powder is lowered through oxidation or corrosion. Furthermore, when a noble metal (e.g., silver), which does not easily undergo oxidation or corrosion, is used for, for example, wires of an IC, etc., the noble metal involves problems, including short circuit due to migration. Although carbon powder does not have such a drawback of metallic powder, the electrical conductivity of carbon powder is lower than that of metallic powder. Therefore, in order to enhance electrical conductivity, there have been proposed, for example, carbon fiber which is easily graphitized and has a specific structure in which an aspect ratio is large (Japanese Patent Publication (kokoku) No. 06-39576), or a material containing entangled carbon fiber filaments (Japanese Patent Application Laid-Open (kokai) No. 07-102197).
However, in the case where the aforementioned electrically conductive material is incorporated into a resin, a problem arises that transparency inherent to the resin may be lost when the incorporation amount of the conductive material is increased in order to enhance the electrical conductivity of the resin. For example, when a material containing entangled carbon fiber filaments is incorporated into a resin, the incorporation amount of the material must be tens of mass % in order to secure sufficient enhancement of the electrical conductivity of the resin. As a result, when the thickness of a coating or a film formed from the resin is about 1 mm, the transmittance of the coating or film becomes about 30%; i.e., the coating or film becomes opaque and barely transmits light. In contrast, when the amount of carbon fiber incorporated into a resin is reduced in order to maintain the transparency of the resin, the electrical conductivity of a coating or film formed from the resin is greatly reduced.
There has also been proposed an electrically conductive transparent composition prepared from an electrically conductive material to which, in order to enhance electrical conductivity, a mixture of graphite having an average particle size of 1–20 μm and carbon powder having a BET specific surface area of 25–800 m2/g has been incorporated (Japanese Patent Application Laid-Open (kokai) No. 2000-173347). However, when the composition is formed to have a thickness of 0.02–0.5 μm and a transmittance of 30%, the surface resistivity of the composition is 1×105 Ω/□ (ohm/square)(or simply referred to Ω, hereinafter the same will do); i.e., the electrical conductivity of the composition is still low. As described above, conventional electrically conductive coating or electrically conductive film encounters difficulty in attaining both transparency and high electrical conductivity.
An object of the present invention is to overcome the aforementioned problems of conventional electrically conductive coating or electrically conductive film and to provide an electrically conductive transparent composition comprising carbon fiber, in particular vapor grown carbon fiber (hereinafter sometimes abbreviated as “VGCF”), of very small outer diameter and high electrical conductivity, which composition does not lose transparency inherent to a resin and exhibits both transparency and high electrical conductivity; and an electrically conductive transparent material formed from the composition.
Vapor grown carbon fiber (VGCF) is produced by thermally decomposing a raw material gas, such as hydrocarbon gas, in a vapor phase in the presence of a metallic catalyst, and by growing the decomposition product into a fibrous shape. It has been known that carbon fiber having a diameter of tens of nm to 1,000 nm can be produced through this process.
A variety of processes for producing VGCF are disclosed, including a process in which an organic compound such as benzene, serving as a raw material, and an organic transition metal compound such as ferrocene, serving as a catalyst, are introduced into a high-temperature reaction furnace together with a carrier gas, to thereby produce VGCF on a substrate (Japanese Patent Application Laid-Open (kokai) No. 60-27700); a process in which VGCF is produced in a dispersed state (Japanese Patent Application Laid-Open (kokai) No. 60-54998 (U.S. Pat. No. 4,572,813)); and a process in which VGCF is grown on a reaction furnace wall by means of spraying onto the furnace wall droplets of a solution containing a raw material and a metallic catalyst (Japanese Patent No. 2778434).
The aforementioned processes have enabled production of carbon fiber of relatively small outer diameter and high aspect ratio which exhibits excellent electrical conductivity and heat conductivity and is suitable as a filler material. For example, carbon fiber having an outer diameter of about 10 to about 200 nm and an aspect ratio of about 10 to about 500 has been mass-produced and used, for example, as an electrically conductive or heat conductive filler material to be incorporated into electrically conductive resin, or as an additive to be incorporated into lead storage batteries.
A characteristic feature of a VGCF filament resides in its shape and crystal structure. A VGCF filament has a multi-layered shell structure having a very thin central hollow portion, wherein a plurality of carbon hexagonal network layers are grown around the hollow portion so as to form annual rings.
A carbon nano-tube, which is a type of carbon fiber having a diameter smaller than that of VGCF, has been discovered in soot obtained by evaporating a carbon electrode through arc discharge in helium gas. The carbon nano-tube has a diameter of 1–30 nm, and has a structure similar to that of a VGCF filament; i.e., the tube has a hollow cylindrical structure having a central hollow portion, wherein a plurality of carbon hexagonal network layers are grown around the hollow portion so as to form annual rings. However, the process for producing the nano-tube through arc discharge is not carried out in practice, since the process is not suitable for mass production.
Meanwhile, carbon fiber of high aspect ratio and high conductivity can be produced through the vapor-growth process, and therefore various improvements to the carbon fiber have been made. For example, U.S. Pat. No. 4,663,230 and Japanese Patent Publication (kokoku) No. 3-64606 (European Patent No. 205556) disclose a graphitic cylindrical carbon fibril having an outer diameter of about 3.5 to about 70 nm and an aspect ratio of at least 100. The carbon fibril has a structure such that a plurality of layers of ordered carbon atoms are continuously disposed concentrically around the longitudinal axis of the fibril, and the C-axis of each of the layers is substantially perpendicular to the longitudinal axis. The entirety of the fibril has a smooth surface, and includes no thermal carbon overcoat deposited through thermal decomposition. Japanese Patent Application Laid-Open (kokai) No. 61-70014 discloses vapor grown carbon fiber having an outer diameter of 10–500 nm and an aspect ratio of 2–30,000, the thermal decomposition carbon layer of the carbon fiber having a thickness of 20% or less the diameter of the carbon fiber. However, detailed studies have not yet been performed on the branched hollow structure, compressed specific resistance, and heat conductivity of the aforementioned carbon fibers.
Carbon fiber has low contact resistance, and, as compared with conventional carbon black or similar material, exhibits excellent electrical conductivity and heat conductivity, and has high strength, since, in carbon fiber, carbon structure is developed along a longitudinal direction of a fiber filament, and fiber filaments are entangled extensively with one another. Therefore, various attempts have been made to enhance such characteristics of carbon fiber. For example, Japanese Patent No. 2862578 (European Patent No.491728) discloses that the contact resistance of carbon fiber is reduced by incorporating, into a resin composition, carbon fiber containing entangled fiber filaments. Japanese Patent No. 1327970 discloses branched VGCF in which fresh VGCF is grown on a VGCF substrate. Japanese Patent Application Laid-Open (kokai) No. 6-316816 discloses VGCF having gnarled depositions thereon.
The aforementioned attempts have been made in order to ensure contact between fine carbon fiber filaments in a composite material, by bringing the filaments into contact with one another or by bonding the filaments with one another in advance. In addition to such carbon fiber filaments, there has been a demand for a single carbon fiber filament of enhanced electrical conductivity or heat conductivity.